1. Field of the Invention
This invention relates to electric discharge machining (EDM) and specifically to an electrode wire to be used in discharge machining and to the process for manufacturing an EDM electrode wire.
2. Description of the Relevant Prior Art
The process of electrical discharge machining (EDM) is well known. An electrical potential (voltage) is established between a continuously moving EDM wire electrode and an electrically conductive workpiece. The potential is raised to a level at which a discharge is created between the EDM wire and the workpiece. The intense heat generated by the discharge will melt and/or vaporize a portion of both the workpiece and the wire to thereby remove, in a very small increment, a piece of the workpiece. By generating a large number of such discharges a large number of increments are removed from the workpiece whereby the workpiece can be cut very exactly to have a desired planar contour. A dielectric fluid is used to establish the necessary electrical conditions to initiate the discharge and to flush debris from the active machining area.
The residue resulting from the melting and/or vaporization of a small increment (volume) of the surface of both the workpiece and EDM wire electrode is contained in a gaseous envelope (plasma). The plasma eventually collapses under the pressure of the dielectric fluid. The liquid and vapor phases created by the melting and/or vaporization of material are quenched by the dielectric fluid to form solid particulate matter or debris. The cutting process therefore involves repeatedly forming a plasma and quenching that plasma. This process may happen at the same time at many spots along the length of the EDM wire.
It is important for flushing to be efficient because, if flushing is inefficient, conductive particles build up in the gap which can create the potential for electrical shorts which can result in arcs. Arcs are very undesirable as they cause the transfer of a large amount of energy which causes large gouges or craters, i.e., metallurgical flaws, to be made in a workpiece or the EDM wire. Such flaws in the wire could cause the EDM wire to break catastrophically.
A composite surface layer on an EDM wire electrode being subjected to erosion is preferably sufficiently thick and tenacious enough to survive the erosion process. It preferably will have a low volumetric heat of sublimation which will allow it to sublime (vaporize) in preference to melting. Sublimation (the phase change of a solid as it transforms directly to a gas or vapor) will produce relatively small particulate debris when the vapor is condensed after being quenched by the dielectric fluid as the plasma envelope is collapsed at the conclusion of each discharge. Larger particulate debris will be produced by melting and will be more difficult to flush away by the hydraulic action of the dielectric fluid. Metals used for the surface layer of any EDM wire would preferably be characterized as having a low energy of sublimation, sometimes referred to as a low volumetric heat of sublimation measured in kilo joules per cubic centimeter (KJ/cm.sup.3). Such metals are well-known in the prior art and include cadmium, bismuth, lead, zinc, tin, antimony or an alloy of those metals. Zinc has a relatively low volumetric heat of sublimation, and alloys of zinc will also have a relatively low heat of sublimation with alloys having the highest zinc content having the lowest volumetric heat of sublimation.
In addition to the physical properties of the exposed surface affecting flushability, the topography of the surface may also affect flushability. For example, a convoluted topography can promote hydraulic turbulence at the surface, thereby improving the flushing action of the dielectric fluid.
An EDM wire must possess a tensile strength that exceeds a desired threshold value to avoid tensile failure of the EDM wire electrode induced by the preload tension that is applied, and should also possess a high fracture toughness to avoid catastrophic failure induced by the flaws caused by the discharge process. Fracture toughness is a measure of the resistance of a material to flaws which may be introduced into a material and which can potentially grow to the critical size which could cause catastrophic failure of the material. The desired threshold tensile strength for EDM wire electrodes is generally thought to be in the range of 60,000 to 90,000 PSI.
Since the EDM wire must also conduct electricity, it is important that the EDM wire is a good conductor of electricity. The main function of the EDM wire electrode is to deliver electrical energy to the gap. Higher conductivity wire performs this function more efficiently. All other factors being equal, the highest conductivity EDM wire will always cut the fastest and therefore will be the most efficient.
The ideal traveling EDM wire electrode will therefore have: an adequately thick and tenacious surface layer with a low heat of sublimation; a tensile strength greater than the threshold value to prevent tensile failures; high fracture toughness; and good electrical conductivity.
It is known in the prior art to use an EDM wire electrode with a core composed of a material having relatively high mechanical strength with a relatively thin metallic coating covering the core and comprising at least 50% of a metal having a low volumetric heat of sublimation such as zinc, cadmium, tin, lead, antimony, bismuth or an alloy thereof. Such a structure is disclosed is U.S. Pat. No. 4,287,404 which discloses a wire having a steel core with a coating of copper or silver which is then plated with a coating of zinc or other suitable metal having a low volumetric heat sublimation.
It is also known from the prior art, for instance from U.S. Pat. No. 4,686,153, to coat a copper clad steel wire with zinc and thereafter to heat the zinc coated wire to cause dispersion diffusion of the copper into the zinc layer to thereby convert the zinc layer into a copper-zinc alloy. That patent describes the desirability of a .beta. phase alloy layer for EDM purposes. The copper zinc alloy has a concentration of zinc of about 45% by weight with the concentration of zinc decreasing radially inwardly from the outer surface. The average concentration of zinc in the copper-zinc alloy layer is less than 50% by weight but not less than 10% by weight. The surface layer therefore includes .beta. phase copper-zinc alloy material at the outer surface since .beta. phase copper-zinc alloy material has a concentration of zinc of 40%-50% by weight. While this patent recognizes that a copper-zinc alloy layer formed by means of a dispersion diffusion heat treatment may contain .epsilon. phase material (zinc content about 80%); .gamma. phase material (zinc content about 65%); .beta. phase material (zinc content about 45%); and a phase material (zinc about 35%) in accordance with Hansen's phase diagram, this patent indicates that the preferred alloy material is .beta. phase alloy material.
Due to the nature of the EDM process, EDM machining is a relatively slow process. In many wire EDM applications, the surface finish of the part being fabricated is of critical importance. In order to achieve superior surface finish on the completed part, many operators will "rough cut" the part to slightly oversized dimensions and subsequently perform multiple (sometimes as many as five to eight) "skim cuts" where they employ reduced power to thereby attempt to remove only a small amount of the surface, i.e. minute bites per discharge. This procedure allows the operator to maintain very tight dimensional control on the part geometry while simultaneously creating a much smoother surface finish. Obviously the number of "skim" passes required to achieve a given surface finish can have a large impact on the cost of a given part, and EDM operators are always seeking ways to achieve improved surface finish at competitive metal removal rates. It is therefore desired to improve the speed of cutting without degrading the surface finish achieved by the cutting process.
Also, many of the more recent EDM wire installations include an "autothreading" system. All wire EDM machines include an upper and lower wire guide system that accurately positions the wire by passing it through a high tolerance wire guide that may be only several microns larger than the actual wire diameter. If the wire breaks during the machining process, or if an internal cavity has been cut and one needs to move to another area to cut additional internal cavities, it is necessary to rethread the wire through these guides to continue the machining operation. These "autothread" systems all operate much more reliably if the wire being threaded through them is very straight. Therefore, it is also desired to provide EDM wire with improved mechanical properties, such as stiffness and straightness, to facilitate reliable "autothreading."